Method of coating iron,nickel or cobalt alloy with aluminum

ABSTRACT

ALLOY IN A NON-OXIDIZING ATOMOSPHERE TOA TEMPERATURE ABOVE ABOUT 1800*F. TO CAUSE THE ALUMINUM-MAGNESIUM MATERIAL TO COAT AND DIFFUSE INTO THE TREATED SURFACE OF THE IRON, NICKEL OR COBALT BASE ALLOY.   A PROCESS FOR COATING AN IRON, NICKEL OR COBALT BASE ALLOY IS PROVIDED WHICH COMPRISES TREATING A SURFACE OF THE SELECTED ALLOY WITH A MATEIAL CONSISTING ESSENTIALLY OF FROM ABOUT 10 TO 30 WEIGHT PERCENT MAGNESIUM WITH THE BALANCE BEING ALUMINUM AND THEN HEATING THE SO-TREATED

0 y ,19 M. e. WHITFIELD L' I 3,577,268 I METHOD OF COATING IRON, NICKELOR COBALT ALLOY WITH ALUMINUM Original Filed Nov. 25, 1964 i z wmsawavwNV 3 iM SL-QZJ l- {5 k. (6) EM: HUM ELfiSJ; 5E3 I (Nwwmawn)V/////////////// LL 2 J J g E s 8' s 2 s s o Maven :IO Noun/mm OJ.sa-loxa swag/w (uouvmawao) THER MAL. FATIGUE TEST BASE MATERIAL- COLBALTBASE A LO (KNOWN COMHERCIALLY As w: 523

TEMPERATURE CYCLE 20o-20oo'i= 8000 IDVZD d0 NOLLVLLONI OJ- SE'IDADEQVHBAV Q INVENTORS' Mars/ml 6'. w n'ifidldl 2/8/74/ 5. Par: ac/mwsk 1'fiJENT United States Patent 3,577,268 METHOD OF COATING IRON, NICKEL ORCOBALT ALLOY WITH ALUMINUM Marshall G. Whitfield, Brookfield, Richard S.Parzuchowski, Danbury, and Dennis B. Moore, Ridgefield, Conn., assignorsto Cabot Corporation Continuation of application Ser. No. 413,771, Nov.25,

1964. This application Mar. 12, 1969, Ser. No. 809,471 Int. Cl. C23c9/02 U.S. Cl. 117-107.2 12 Claims ABSTRACT OF THE DISCLOSURE A processfor coating an iron, nickel or cobalt base alloy is provided whichcomprises treating a surface of the selected alloy with a materialconsisting essentially of from about to 30 weight percent magnesium withthe balance being aluminum and then heating the so-treated alloy in anon-oxidizing atmosphere to a temperature above about 1800 F. to causethe aluminum-magnesium material to coat and diffuse into the treatedsurface of the iron, nickel or cobalt base alloy.

This application is a continuation of application Ser. No. 413,771,filed Nov. 25, 1964, now abandoned.

The present invention relates to coating of metals such as iron, nickeland cobalt base alloys commonly used for gas turbine blading, combustionliners and fuel-air nozzles among other parts. Oxidation and hightemperature corrosion and erosion resistance above that possessed by thebasic alloys is, of course, the main reason why coatings are necessarilywidely used.

One of the best known and useful coatings is produced by diffusingaluminum into the surface of engine parts to a depth of severalthousandths of an inch, thereby creating a new alloy with the metalscomprising the basic alloy of which the engine part is made.

Such a coating may be made in a variety of Ways, such as hot dip,thermal decomposition, electrophoretic deposition and chemicaldisplacement to mention the best known. In some cases, chromium,silicon, cobalt, nickel and iron have been used with the aluminum in aneffort to improve the basic characteristics of the new alloy beingcreated in or on the surface of the parts being treated.

In the past such engine parts have comprised nickel, cobalt or ironalloys, and the table which follows gives as an example the compositionof typical alloys in this category.

Cobalt alloy Nickel alloy (Waspalloy) 1 Maximum.

While such prior coatings have served well and represented aconsiderable improvement over uncoated metal parts, there are definitelimitations with respect to the erosion resistance and also the uppertemperature limit at which the parts may be safely operated. This istrue even with close control of the coating process and under iceoptimum conditions; the products thus coated have bounds which, as thetechnology of engines advances, are considered to be less and lesssatisfactory.

In particular, there is a need for the best erosion protection andresistance to thermal fatigue that can possibly be developed in alloymetals.

Accordingly, an object of the present invention is to provide animproved coating and an improved coated product, as well as an improvedmethod for treating or coating heat-resistant metals and alloys,particularly the alloys set forth above and additionally a packcementation powder mixture associated therewith, by which the upperlimits of thermal fatigue are markedly increased without significantdetriment to erosion resistance.

Another object of the invention is to provide an improved coated ordiffused product, process and treating or coating agent as above setforth, which may be economically carried out or produced both from thestandpoint of cost of materials and processing time, as well as from thestandpoint of equipment needed.

A feature of the invention resides in the provision of an improvedcoating method and coating agent which are not especially critical butinstead exhibit complete utility under conditions which are consideredcommercially acceptable for production purposes.

In the drawings:

FIG. 1 is a bar graph involving thermal fatigue, comparing specimenstreated in accordance with the invention, with prior art specimens, thebase material being a cobalt-base alloy.

FIG. 2 is a bar graph involving thermal fatigue, comparing nickel-basealloy specimens treated in accordance with the invention, with prior artspecimens.

We have found that, contrary to what might be expected, theincorporation of very small amounts of the metal magnesium in analuminum or aluminum alloy coating or diffusion, wherein the variousmetals involved are alloyed at and with the surface portions of the basealloy structure, provides a surprisingly improved product with respectto thermal fatigue without appreciable detriment to erosion resistance.

Of itself, magnesium is known to be a highly inflammable, unstable metalwhich reacts violently when its temperature is raised, being unable towithstand elevated temperatures in the manner of most other metals ofthe temperature-resistant class. Such undesirable properties ofmagnesium would lead one to believe that its addition or incorporationin an alloy coating for the purpose of increasing the resistance tothermal fatigue would have anything but a beneficial effect.

By actual tests, however, we have found that coatings and diffusions ofaluminum or aluminum alloy wherein aluminum is the dominant material,which have very small percentages of magnesium incorporated, changegreatly for the better the characteristics of the treated part, by whichhigh temperatures and thermal fatigue are resisted. The improvementeffected has been determined from a number of actual, repeated testsmade on sample alloy pieces which include iron, nickel or cobalt bases.

We have determined that in certain samples wherein the coating orsurface diffusion alloy which contains aluminum also includesapproximately from 2% to 6% of magnesium at surface portions of thespecimen, there is an increase of resistance to fatigue of roughly from30% to 50%, without significant detriment to erosion resistance.

The coating or diffusion alloy may with benefit include small amounts ofthe refractory metal group, including columbium, tantalum, molybdenumand tungsten, as well as small amounts of titanium and chromium. Suchcoatings and diflusions were employed on alloy parts having metals ofthe iron group, namely iron, nickel and cobalt.

The results are borne out by the bar graphs of FIGS. 1 and 2. Thespecimens comprised the iron base, as well as cobalt base and nickelbase alloys. The test pieces were subjected to a temperature cycle of200 -2000 F. During the cycle the pieces were heated to 2000 F. forforty seconds, then blasted with air for twenty seconds followed bynon-agitated air cooling for an additional twenty seconds.

Specimens were also subjected to erosion tests, involving exposures for100 hours at 2000 F. in an oxidizing atmosphere. The specimens passedsuch tests with weight losses of not more than 0.08 gram which is wellwithin the allowable limit set by turbine engine manufacturers. Theinitial weight of the specimens in each instance was in the neighborhoodof 50 grams.

The treatment or coating and diffusion of the base alloy structure maybe done by means of a pack cementation powder mixture or else it may bedone by dipping the base alloy structure in a molten bath containing analloy of aluminum and magnesium and thereafter heat treating the alloystructure after removal from the bath.

The following examples are given of pack cementation powder mixtureswhich have been utilized in the firstmentioned method and which haveproduced coated articles of superior characteristic as alreadyindicated.

EXAMPIJE 1 Cementation mix of 98.5% of commercial grade aluminum oxideparticles having a size of 220 mesh, 1.5% of aluminum-magnesium alloy(10% magnesium) powder having a size to pass through a 20 mesh screen,to which is added 35 grams/100 pounds of hydrazine dihydrochloride as anactive gas-producing ingredient. This is packed about the parts to betreated and is heated to 1910 F. i|10% for two hours and allowed tocool. The parts are removed, washed and found to have approximately twomils of alloy coating all over.

EXAMPLE 2 Cementation mix of approximately 98% of aluminum oxideparticles having a size of 220 mesh, 1.5% of aluminum powder of size 325mesh, and .15 of magnesium powder of size 325 mesh, to which is added 35grams/100 pounds of hydrazine dihydrochloride. The procedure for packingthe treating is as given in Example 1.

EXAMPLE 3 Cementation mix: 98.5% of pure A1 particles having a size of220 mesh, 1.5 of aluminum-magnesium alloy (10% mg.) particles having asize of 90 mesh, to which is added hydrazine dihydrochloride in theproportion of 35 grams/100 pounds. The procedure followed is as given inExample 1.

EXAMPLE 4 A cementation mix consisting of 97% of commercially purealuminum oxide particles having a size of 220 mesh, 1 /2 ofaluminum-magnesium alloy particles (30% magnesium) having a size of 325mesh, 1 /2% of aluminum powder having a size of 325 mesh, to which isadded 35 grams/100 pounds of hydrazine dihydrochloride as an activegas-producing ingredient. Subsequent procedures are as above.

An example of a method involving dipping the pieces in a molten bath isas follows:

EXAMPLE 5 A bath of aluminum-magnesium alloy, containing to magnesiumheld at 1320 F. to 1375 F. is used to coat an engine blade or vane, asby dipping a prepared piece in the bath. After coating, the excessmolten metal is removed by brushing, vibrating or otherwise. The initialcoating is then diffused to form the final alloy coating by heating thepiece in a neutral or reducing atmosphere for approximately four hoursat 1975 F. to give a final alloy thickness of .003 all over. Diffusionwill vary with 4 the base alloy heat treatment and the temperature towhich the part is subjected.

A careful analysis of coated cobalt-base alloy structures treated by theabove methods has revealed the following make-up of the surface areasand areas immediately under the surface.

SPECIMEN #1 On surface Percent Al 50-55 Co 18-20 Cr 5-7 W 1-2 Also, someZnO and A1 0 on surface.

At surface after slight buffing Percent Fe l-Z Mg 2-3 No zinc.

SPECIMEN #2 On surface Percent Al 45-50 Cr i 1-2 W 4 4-5 Ti 2-3 Fe0.2-0.5 Mg 4-6 Also, A1 0 on surface and no Zn present.

At surface after slight bufling Percent Al 28-32 Co 47-51 Cr 12-15 W 5-7Ti 1-2 Fe 1-2 It will be noted that the surface portions of thespecimens are constituted of an alloy comprised of the base alloy,aluminum in quantities of roughly 50% and magnesium in quantities ofseveral percent, being indicated as in the range of from 2% to 6%, thesefigures being non-critical in that somewhat smaller or largerpercentages also provide an improved product but with differentcharacteristics from those indicated in the specified tests.

The above analysis was made with an electron beam microanalyzer usingX-rays and a single crystal X-ray spectrometer which provides a spectrumanalysis. This equipment has revealed the chemistry of the outer surfaceof samples coated according to the invention, as well as that of themetallic coating layers just beneath the outer surface, which layer isexposed by subjecting the samples to a slight buffing.

The base structure of the specimens comprised a cobalt base alloy knowncommercially as W1-52 and having approximately 20% to 22% chromium, 10%to 12% tungsten, .6% iron and the balance cobalt.

The presence of cobalt and nickel and iron is explained because theseare derived from the cobalt base, nickel base or iron base alloysthemselves. Titanium and chromium occur, as well as metals of therefractory group,

which group includes niobium, columbium, tantalum, molybdenum andtungsten, and it is at present believed that some metals of the abovelist are due to impurities in the aluminum and mixes used in thespecific coating procedure that was analyzed, and that they contributebeneficially tothe improved end result.

In addition to the cementation and dip procedures, a slurry may beutilized. Such a slurry involves particles in suspension and a binder,by which the parts are coated prior to heat treatment. The slurry maycomprise an alloy powder of aluminum magnesium constituted, for example,of 90% aluminum and 10% magnesium. From 25 to 75 parts by weight of thispowder may be used with 75 to 25 parts of flux, such as a fiuxingsubstance known commercially as Cryolite. Cryolite may be made up of 40%of KCl, 40% NaCl, 6% lithium fluoride and the balance comprising sodiumfluoride and aluminum fluoride in the form of 3NaFAlF From 40% to 80% ofa liquid vehicle and 2% of a binder may be used with the novel packcomposition provided by the invention. and thereafter heat-treated attemperatures sufficiently high to diffuse and alloy the aluminum andmagnesium into the parts.

As at present understood, the improved results obtained in the specimensby practicing the invention are the consequence of alloying smallamounts of magnesium with aluminum or aluminum alloys and with the basealloy of the specimens, whether such base alloy is of the cobalt type,or nickel or iron type. The specific examples given above of thecementation packs are merely illustrative of the novel pack compositionprovided by the invention. The method of the invention, embracing thealloying with magnesium by either a cementation pack, molten bath or aslurry, has also been explained, as well as the improved productcomprising the coated and diifused pieces and the chemical make-upthereof involving the alloying of magnesium and aluminum, or aluminumalloy.

As already noted, the invention is of particular importance in theproducing of parts such as vanes or blades, nozzles, combustion linersand the like for use with gas turbines where oxidation, high temperaturefatigue and erosion represent major factors in the operation. Theinvention has utility in the production of coated and diffused oralloyed parts for all types of high temperature applications, as will beunderstood.

In accordance with the invention, a machine part such as a piece formedof Waspalloy (nickel) alloy may be advantageously first coated ordiffused with chromium, and thereafter have the magnesium-containingcoating applied. A cementation pack may be initially used, whichincludes powdered ferro-chromium alloy (65% Cr and 35% Fe) in a quantityof approximately 65% by weight, about I /2% of ammonium fluoride NH Fand alumina owder such as the oxide A1 to make up the balance(approximately 33 /2 by weight). With the pack at about 1850 F. for fiveor six hours a chromium deposit in the neighborhood of .001" to .002"will be etfected. Thereafter the processing to effect the coatingcontaining magnesium may be done, as explained above.

If the presence of iron is considered deleterious in the treated piecedue to specialized requirements, the cementation pack may utilizechromium in another form which is devoid of iron, as will be understood.

An iron-containing cementation powder mixture which may beadvantageously utilized, in accordance with the invention, comprisesapproximately 40% ferro-aluminum, approximately 40% aluminum oxide, notmore than 20% magnesium oxide and traces of a gas-producing agent suchas hydrazine dihydrochloride. Such mixture efliects a diffusioncontaining magnesium, as with the other examples given above.

In the light of the present invention and disclosure, variation andmodifications to meet the individual whim or particular need willdoubtless become evident to those skilled in the art, to obtain all orpart of the benefits outlined above without copying exactly the specificexamples given herein, and we therefore claim all such insofar as theyfall within the reasonable spirit and scope of the appended claims.

What is claimed is:

1. The method of coating a metal alloy structure having an iron ornickel or-cobalt base, which comprises treating the structure withexclusion of air at a temperature of more than 1800 F. with a source ofboth aluminum and magnesium wherein the magnesium is present inquantities ranging from about 10 to 30 weight percent with the balanceessentially aluminum.

2. The method of coating a metal alloy structure having an iron ornickel or cobalt base, which comprises treating the structure with theexclusion of air at a temperature of approximately 1800 F. to 2000 F.with a material consisting essentially of a small amount of finelydivided particles of aluminum and magnesium, and an inert material andtraces of a source of gaseous halogen, wherein said finely dividedparticles are present in the amount of between 10% and 30% by weightmagnesium and the balance essentially aluminum.

3. The method of coating a metal alloy structure having an iron ornickel or cobalt base, which comprises treating the structure withexclusion of air at a temperature of approximately 1800 F. to 2000 F.with a mixture of finely divided particles comprising aluminum oxide andaluminum magnesium alloy, and traces of a source of gaseous halogen,wherein the weight percent of magnesium based on the entire mitxure isfrom about .15 to 1% and said weight percent aluminum oxide is at least97% based on the entire mixture.

4. The method of coating a metal alloy structure having an iron ornickel or cobalt base, which comprises treating the structure withexclusion of air at a tem, perature of approximately 1800 F. to 2000 F.with a mixture of finely divided particles comprising aluminum oxide,aluminum powder and magnesium powder and traces of a source of gaseoushalogen, wherein the weight percent of magnesium based on the entiremixture is from about .15 to 1% and the weight percent of aluminum oxideis at least 97% based on the entire mixture, the balance being aluminumpowder.

5. In a process of applying an aluminum magnesium coating to a metalalloy structure having an iron or nickel or cobalt base by the steps ofsurrounding said structure with the donor material, which materialincludes a small amount of active constituents consisting of magnesiumin a weight percent of from about 10 to 30 with the balance beingessentially aluminum, traces of a source of gaseous halogen and an inertmaterial; and heat treating the said structure in an atmosphere freefrom oxygen to a temperature of at least 1800 F.; the combination inwhich the active constituents of the donor material comprise an alloy ofaluminum and magnesium.

6. In a process of applying an aluminum magnesium coating to a metalalloy structure having an iron or nickel or cobalt base by the steps ofsurrounding said structure with the donor material, which materialincludes a small amount of active constituents consisting of magnesiumin a weight percent of from about 10 to 30 with the balance hereinessentially aluminum traces of a source of gaseous halogen and an inertmaterial; and heat treating the said structure in an atmosphere freefrom oxygen to a temperature of at least about 1800" F.; the combinationin which the active constituents of the donor material comprise a powderof aluminum and magnesium.

7. In a process of applying an aluminum magnesium coating to a metalalloy structure having an iron or nickel or cobalt base by the steps ofsurrounding said structure with the donor material, which materialincludes essentially magnesium in a weight percent of from about 10 to30 with the balance being essentially aluminum, the combination in whichthe donor material comprises a molten alloy 'bath of aluminum andmagnesium held at a temperature in the neighborhood of 1300 F.; and heattreating the said structure in an atmosphere free from oxygen to atemperature of at least 1800 F.

8. The combination of claim 5, wherein the inert material is aluminumoxide.

9. The method of coating an iron, nickel or cobalt base alloy structure,which comprises depositing chromium on a surface of said alloy and thentreating the structure with exclusion of air at a temperature ofapproximately 1800 F. to 2000 F. with a mixture of powders containing asingredients an inert material, a source of halogen gas and a smallquantity of the metals aluminum and magnesium, wherein the aluminum andmagnesium metal constituents consist essentially of 10% to 30% by weightmagnesium and the balance aluminum.

10. The method of coating a metal alloy structure having an iron ornickel or cobalt base which comprises treating the structure with theexclusion of air with a material consisting essentially of particles ofaluminum and magnesium, an inert material and traces of a source ofgaseous halogen, at a temperature sufliciently high to diffuse and alloyaluminum and magnesium into the structure, said particles beingoriginally present in an amount between 10% and 30% by weight magnesiumand the balance essentially aluminum.

11. The method of coating a metal alloy structure having an iron ornickel or cobalt base which comprises treating the structure withexclusion of air with a mixture of finely divided particles comprisingaluminum 30 oxide and aluminum magnesium alloy, and traces of a sourceof gaseous halogen, wherein the weight percent of magnesium based on theentire mixture is from about 0.15% to 1% and said weight percent ofaluminum oxide 8 is at least 97% based on the entire mixture, andtreating said structure with said mixture at a temperature sulficientlyhigh to cause diffusion of aluminum and magnesium into the structure.

12. The method of coating a metal alloy structure having an iron ornickel or cobalt base which comprises treating the structure with theexclusion of air with a mixture of finely divided particles comprisingaluminum oxide, aluminum powder and magnesium powder, and traces of asource of gaseous halogen, wherein the weight percent of magnesium basedon the entire mixture is from about 0.15% to 1% and the weight percentof aluminum oxide is at least 97% based on the entire mixture, thebalance being the aluminum powder, and treating said structure with saidmixture at a temperature high enough to cause diffusion and alloying ofaluminum and magnesium into the structure.

References Cited UNITED STATES PATENTS 1,706,130 3/1929 Ruder. 2,300,40011/ 1942 Axline. 2,406,245 8/ 1946 Oganowski et al. 2,664,874 l/1954Graham. 2,752,265 6/ 1956 Whitfield et al. 2,774,686 12/1956 Hodge117-114 3,026,606 3/ 1962 Nickola 117--114X 3,055,711 9/1962 Sprowl117--114 RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

